Borehole apparatus and methods for measuring formation velocities as a function of azimuth, and interpretation thereof

Abstract

Borehole tools are provided with at least one transmitter which generates acoustic waves at a plurality of azimuthal locations about the borehole, and at least one receiver which receives and measures a characteristic (e.g., velocity) of the acoustic waves at related azimuthal locations. The direction of minimum velocity around the borehole is considered the direction of maximum uniaxial stress in the formation. From the velocity as a function of azimuth information, determinations of formation properties, and logs of the same can be made. The azimuthal direction of minimum velocity around the borehole predicts the propagation direction of artifically induced hydrofractures. The velocity variation around the borehole at a particular depth of the borehole is taken as an indication of susceptibility to failure, with higher velocity variations indicative of a more poorly consolidated formation or a formation with a large uniaxial stress. The curvature of the velocity versus stress curve in the formation is also indicated by how poorly a sine wave fits to the velocity data. Other parameters of the formation are obtained by fitting a best fit curve to the azimuth versus velocity data, where adjustable parameters of the best fit curve constitute the formation parameters.

I claim: 
     
       1. Apparatus for investigating earth formations traversed by a borehole, comprising: a) a transmitter for generating acoustic waves at a plurality azimuths about the borehole and for transmitting the acoustic waves into the earth formations;   b) a receiver axially spaced from the transmitter for receiving, at the plurality of azimuths, the acoustic waves transmitted into the formations near the borehole at the plurality of azimuths; and   c) a processor coupled to the receiver for determining acoustic velocities of the formations around the borehole as a function of azimuth and finding the azimuth of one of a maximum acoustic velocity and a minimum acoustic velocity and generating at least one indication relating to the earth formations as a result thereof.   
     
     
       2. Apparatus as claimed in claim 1, wherein the transmitter comprises a segmented cylindrical transmitter which generates compressional waves. 
     
     
       3. Apparatus as claimed in claim 2, wherein the receiver comprises a segmented cylindrical receiver. 
     
     
       4. Apparatus as claimed in claim 1, wherein the receiver comprises at least two axially spaced receivers. 
     
     
       5. Apparatus as claimed in claim 4, wherein each of the at least two axially spaced receivers are segmented cylindrical receivers. 
     
     
       6. Apparatus as claimed in claim 1, wherein the apparatus further comprises a sonde having extensible arms and pads pending from the extensible arms, the pads being azimuthally spaced around the borehole, the transmitter comprising a plurality of azimuthally spaced transmitters, the receiver comprising a plurality of azimuthally spaced receivers, and each pad including at least one of the plurality of transmitters and one of the plurality of receivers. 
     
     
       7. Apparatus as claimed in claim 6, wherein each pad includes at least two receivers which are axially spaced from each other at a substantially identical azimuth. 
     
     
       8. Apparatus as claimed in claim 6, wherein each pad includes a plurality of azimuthally spaced transmitters and a plurality of azimuthally spaced receivers, the receivers and transmitters being located at corresponding azimuths. 
     
     
       9. Apparatus as claimed in claim 1, wherein the transmitter and the receiver are (i) directed at the formation, (ii) located at a substantially identical azimuth, and (iii) rotatable in the borehole to a plurality of azimuths about the borehole. 
     
     
       10. Apparatus as claimed in claim 1, wherein the processor further includes means for generating a log of velocity versus azimuth around the borehole at a depth in the borehole. 
     
     
       11. Apparatus as claimed in claim 1, wherein the processor further includes means for generating a log of the azimuth of one of a maximum velocity and a minimum velocity at a plurality of depths in the borehole, the log constituting the at least one indication. 
     
     
       12. Apparatus as claimed in claim 1, wherein the processor further includes means for finding both maximum velocity and minimum velocity at a depth in the borehole and for finding a difference between the maximum velocity and minimum velocity at that depth in the borehole. 
     
     
       13. Apparatus as claimed in claim 12, wherein the processor further includes means for generating a log of differences between maximum and minimum velocities over a plurality of depths in the borehole. 
     
     
       14. Apparatus as claimed in claim 1, wherein the processor further includes means for finding maximum velocity and minimum velocity at a depth in the borehole, determining any difference therebetween, and dividing the difference by one of the maximum velocity and the minimum velocity at that depth in the borehole to provide a percentage velocity variation at that depth in the borehole. 
     
     
       15. Apparatus as claimed in claim 1, wherein the processor further includes means for fitting a sine wave to the acoustic velocities which are a function of azimuth. 
     
     
       16. Apparatus as claimed in claim 15, wherein the processor further includes means for indicating closeness of fit of the sine wave to the acoustic velocities. 
     
     
       17. Apparatus as claimed in claim 1, wherein the processor further includes means for fitting a curve to the acoustic velocities which are a function of azimuth. 
     
     
       18. Apparatus as claimed in claim 17, wherein the curve is defined according to   V(Θ)=V.sub.min +(V.sub.max -V.sub.min)[(1-cos (2Θ+Φ))/2].sup.1/n     where Φ is the azimuth of the minimum of the acoustic velocities about the borehole, and n is a parameter of the formation, V min  is a minimum velocity of the acoustic velocities, and V max  is a maximum velocity of the acoustic velocities.   
     
     
       19. Apparatus as claimed in claim 17, wherein the curve is defined according to   V(Θ)=V.sub.min +(V.sub.max -V.sub.min)[(1-e.sup.2n(1-cos (2Θ+Φ)))/(1-e.sup.4n)]     where Φ is the azimuth of the maximum of the acoustic velocities, and n is a parameter of the formation, V min  is a minimum velocity of the acoustic velocities, and V max  is a maximum velocity of the acoustic velocities.   
     
     
       20. A method for investigating earth formations traversed by a borehole utilizing a borehole tool having a transmitter, a receiver axially spaced from the transmitter, and a processor coupled to the receiver, the method comprising: a) generating acoustic waves with the transmitter at a plurality azimuths about the borehole and transmitting the acoustic waves into the earth formations about the borehole;   b) detecting the acoustic waves at the plurality of azimuths with the receiver;   c) determining acoustic velocities of the formation around the borehole as a function of azimuth with the processor, finding the azimuth of one of a maximum acoustic velocity and a minimum acoustic velocity, and generating at least one indication relating to the earth formations based thereon.   
     
     
       21. A method as claimed in claim 20, wherein the indication is recorded at a plurality of depths in the borehole. 
     
     
       22. A method as claimed in claim 20, further comprising obtaining a velocity variation by comparing a maximum velocity with a minimum velocity. 
     
     
       23. A method as claimed in claim 22, further comprising generating a log of the velocity variation over a plurality of depths in the borehole. 
     
     
       24. A method as claimed in claim 20, further comprising determining any difference between maximum and minimum velocities and dividing the difference by one of the maximum velocity and the minimum velocity to provide a percentage velocity variation. 
     
     
       25. A method as claimed in claim 20, further comprising fitting a sine wave to the acoustic velocities which are a function of azimuth. 
     
     
       26. A method as claimed in claim 25, further comprising determining closeness of fit of the sine wave to the acoustic velocities. 
     
     
       27. A method as claimed in claim 20, further comprising fitting a curve to the acoustic velocities which are a function of azimuth, the curve being defined according to   V(Θ)=V.sub.min +(V.sub.max -V.sub.min)[(1-cos (2Θ+Φ))/2].sup.1/n     where Φ is the azimuth of the minimum of the acoustic velocities about the borehole, and n is a parameter of the formation, V min  is a minimum velocity of the acoustic velocities, and V max  is a maximum velocity of the acoustic velocities.   
     
     
       28. A method as claimed in claim 20, further comprising fitting a curve to the acoustic velocities which are a function of azimuth, the curve being defined according to   V(Θ)=V.sub.min +(V.sub.max -V.sub.min)[(1-e.sup.2n(1-cos (2Θ+Φ)))/(1-e.sup.4n)]     where Φ is the azimuth of the minimum of the acoustic velocities about the borehole, and n is a parameter of the formation, V min  is a minimum velocity of the acoustic velocities, and V max  is a maximum velocity of the acoustic velocities.